postinst is in libpam-cgm 0.39-2ubuntu5.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 | #! /bin/sh
set -e
pam-auth-update --package
exit 0
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